1) Field of the Invention
The present invention relates to a cold cathode and a cold cathode discharge lamp, and a method for producing the cold cathode and the cold cathode discharge lamp.
2) Description of the Related Art
A cold cathode discharge lamp has a remarkably long service life, and there is an increasing demand for cold cathode discharge lamps as backlight sources for liquid crystal displays. Such cold cathode discharge lamps can be classified into two types: an external electrode type and an internal electrode type. FIG. 7 is a partially broken front view that schematically depicts the configuration of a cold cathode discharge lamp of the conventional external electrode type. The cold cathode discharge lamp 101 of the external electrode type has a fluorescent film 103 formed on inner wall surfaces of a glass valve 102. The glass valve 102 is filled with rare gases, and is then hermetically sealed at both ends. A pair of band-like electrodes 104 is formed on outer wall surfaces of the sealed glass valve 102, and the band-like electrodes 104 each having substantially the same length as the glass valve 102 are located opposite to each other. When an AC power supply is connected to the pair of band-like electrodes 104 of the cold cathode discharge lamp 101, and an AC voltage is applied thereto, a dielectric barrier discharge is caused with the glass valve 102, serving as cathodes, that is an insulator (a dielectric) located under the respective band-like electrodes 104. Then, an electric discharge is caused by the rare gases in the inner space of the glass valve 102 between the two band-like electrodes 104. As a result, the fluorescent film 103 in the glass valve 102 is excited to emit visible light. This technique is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 8-236083, for example.
Another type of such a cold cathode discharge lamp is the internal electrode type. FIG. 8 is a cross-sectional schematic view of the cold cathode discharge lamp of the conventional internal electrode type. This cold cathode discharge lamp contains a discharge gas sealed in a transparent long glass tube 110 that has an inner wall with a fluorescent film thereon. The glass tube 110 is hermetically sealed at both ends by stems 111 and 112 to which lead lines 113 and 114 are attached, respectively. The portions of the lead lines 113 and 114 protruding inside the glass tube 110 each have a configuration in which a diamond member 117 (118) with conductivity is fixed to a metal material 115 (116) such as Ni. Thus, the diamond members 117 and 118 and the metal materials 115 and 116 form cathodes 119 and 120, respectively. Unlike the cold cathode discharge lamp of the external electrode type having insulators (dielectrics) as cathodes, the cold cathode discharge lamp of the internal electrode type uses a conductive material for the cathodes. An AC power supply 122 is connected to the lead lines 113 and 114 leading to the cathodes 119 and 120, respectively, and thus, an AC voltage is applied. The ionized gas in the glass tube 110 then collides with the cathodes 119 and 120, and electrons are emitted from the cathodes 119 and 120. These electrons further ionize the gas. This cycle is repeated to have a snowball effect to cause an electric discharge. The fluorescent film 121 in the glass tube 110 is then excited to emit visible light. Here, the diamond exhibits a negative electron affinity or a very low electron affinity, and has a very high secondary emission efficiency accordingly. The diamond also excels in resistance to sputtering. In view of these facts, the conductive diamond members 117 and 118 are used as part of the cathodes 119 and 120, respectively, so that a cold cathode discharge lamp of an internal electrode type that has a long service life and high luminous efficiency can be obtained. The luminous efficiency represents the ratio of the emission luminance to power consumption. Such a cold cathode discharge lamp is disclosed in JP-A No. 2002-298777, for example.
The cold cathode discharge lamps are often used as the backlights for liquid crystal displays. In recent years, more cold cathode discharge lamps are being used for Liquid crystal display (LCD) television sets than for the liquid crystal displays of personal computers. In the case of a liquid crystal display of a personal computer, one cold cathode discharge lamp is used in one liquid crystal display. In the case of a LCD television set, however, ten to twenty of cold cathode discharge lamps are required, because much higher luminance is required than in a LCD display of a personal computer. To operate a cold cathode discharge lamp, an inverter circuit is required. In the case of a LCD television set, it is preferable to connect a number of cold cathode discharge lamps in parallel to an inverter circuit, rather than preparing an inverter circuit for each of the cold cathode discharge lamps, in terms of the size of the product and the production costs.
In view of the facts, the cold cathode discharge lamp of the external electrode type shown in FIG. 7 is advantageous in that the portions of the glass tube located under the external electrodes can be used as ballast capacitors to stabilize an electric discharge, and a number of such cold cathode discharge lamps can be readily connected in parallel to an inverter circuit. However, as the cathodes are made of glass, the luminous efficiency might not be as high as that of the cold cathode discharge lamp of the internal electrode type that has conductive materials with a high secondary emission efficiency provided as cathodes in the glass tube as shown in FIG. 8. Meanwhile, the start and maintenance of an electric discharge in a cold cathode discharge lamp depend on secondary electrons that are emitted when the ions collide with the cathodes. When the cathodes are made of glass that has a low efficiency of emitting secondary electrons upon collision of one ion, the voltage required for the start and maintenance of an electric discharge is high, and as a result, the power consumption becomes large.
On the other hand, the cold cathode discharge lamp of the internal electrode type shown in FIG. 8 is advantageous in having higher luminous efficiency than the cold cathode discharge lamp of the external electrode type. However, an inverter circuit needs to have the same number of ballast capacitors as the cold cathode discharge lamps to be connected to the inverter circuit. Moreover, there are variations in luminance among the cold cathode discharge lamps, because of the problem with stray capacitance of the ballast capacitors and wiring in the inverter circuit. As a result, there might be a case where only two of the cold cathode discharge lamps, at the most, can be connected in parallel to an inverter circuit in practice.